This invention relates to improvements in a method of batch type sterilization by heating which comprises packing a substance to be sterilized such as foodstuff in flexible containers the innermost layers of certain portions thereof being made of films having heat sealing property or in containers which are liable to be broken due to the pressure difference between inside and outside of the containers, charging such packages into a sterilizing tank or retort, admitting steam into the retort for heating packages at a predetermined processing temperature for a predetermined interval for sterilizing the packages and admitting cooling water and compressed air in the retort for cooling the packages. In such a method of sterilization it is necessary to decrease the heating up time of the sterilization process which is necessary to prevent deterioration of the foodstuff and to decrease the time necessary for one cycle of processing. A decrease in the cooling time is also desirable to increase the operating efficiency.
The condition necessary for sterilization is generally determined by taking into consideration the critical point of heating (usually at the center) of the substance being sterilized. Since in most foodstuffs the temperature distribution therein is not uniform when they are heated by steam, a large temperature difference occurs between their surfaces and centers, since the former is heated directly. For this reason, when the surface of the foodstuff is heated to the sterilizing temperature, the center thereof is not yet heated to the required sterilizing temperature whereas its surface is subjected to a relatively large heat hysteresis thus deteriorating the the quality of the foodstuff. Thus, it is also desirable to decrease the heating up time.
In so-called "retort foodstuffs" packed in plastic films and canned goods which can be sterilized in a retort at a high temperature and in a short time, it is necessary to increase the sterilizing temperature to decrease the sterilizing interval for the purpose of preventing the contents from deteriorating. An increase in the sterilizing temperature, however, increases the cooling time thereby increasing the time necessary for one cycle of the operation. Accordingly, it has been desired to decrease the heating up time and the cooling time when a high sterilizing temperature is used.
More particularly, if the same quantity of steam per unit time were admitted into the retort when the sterilizing temperature is increased, the heating up time between the admission of the steam and a point at which the temperature reaches the required sterilizing temperature would be prolonged thereby nullifying the advantage of utilizing high temperature for a short time.
The heating up time may be decreased by rapidly admitting a large quantity of steam into the retort in a short time. In the prior art method and apparatus, however, since the steam was supplied from a single boiler, it has been necessary to use a boiler having a large capacity. Moreover, since it is the recent trend to use a retort of large capacity and to connect a number of retorts in parallel with a single boiler, it is necessary to increase the capacity of the boiler. Under these circumstances, the actual sterilizing temperature is limited to about 120.degree. C with the result that it has been necessary to use a relatively long heating up time of from 7 to 12 minutes.
Further, according to the prior art method and apparatus, cooling of the sterilized packages or cans has been accomplished by sprinkling water thereon and discharging the water collected in the retort by causing the water to overflow. When the cooling water is sprinkled from above, the steam in the retort rapidly condenses by the intimate contact with the cooling water thus causing a large drop in the internal pressure. If air is not admitted into the retort at a sufficiently high rate to prevent excessive pressure drop the packages or can will be ruptured due to a large difference in the pressure on the inside and outside thereof. To eliminate this difficulty, the control valve in a conduit for supplying the cooling water is opened only a little to limit the quantity of the cooling water. This, of course, increases the cooling time, deteriorates the contents of the packages and decreases the operating efficiency. When the cooling water is sprinkled from above as described above, as it reaches the bottom of the retort, its temperature will increase to such an extent that its cooling effect has been substantially lost, whereby the temperature distribution in the retort is not uniform, and only when the retort is filled with the cooling water and it begins to overflow, does a substantial cooling effect begin. Of course, the higher the sterilizing temperature, the longer the cooling time.